Y12 electricity questions

4 - Potential Gradient

Electric Fields Past Paper Questions

Y12 Electric Power

Power and Energy

A 3 kW immersion heater is designed for use on 240 V mains. Find (a) the current taken from the mains, (b) the resistance of the heater, and (c) the energy used in 1 min,


A 2 kW electric fire is designed for use on 250 V mains. What current does it take, and what is its resistance in use?

Question 1 asks `what is its resistance IN USE', Explain why the last two words have been added.


How much energy is stored in a 12 V car battery of capacity 38 A h ? (ie it is capable of delivering 38A for 1 hour before it is flat)

A current of 2 A flows through a resistor of 10 Ω for 2 min, how much energy is used?


How many lamps marked 175 W 240 V could be used in a circuit fitted with a 5 A fuse if they are connected in parallel? (Remember all components in parallel have the same voltage)

An immersion heater marked 3 kW is designed for use on 240 V. If its resistance remains constant, what is its power if connected to a 200 V supply?


A fire of resistance 30 Ω is connected to a 240 V supply by (a) cables of negligible resistance, (b) cables of total resistance 2 Ω. Find the power consumed by the fire in each case.

A 60 W heater is to be made from manganin wire with a resistance of 3.75 ohms per metre. What length of wire is needed if the heater is to operate on a 12 V supply?

Extra
Find the power wasted in the cables when 120 kW is supplied through cables of resistance 0.2 Ω (a) at 240 V, (b) at 200 000 V.
( Hint answer for (a) is 50 kW, and (b) is much smaller !)


Answers

1 (a)     P = VI               P power in W

                                    V p.d. in V       

                                    I current in A

3000 = 240 x I

I=3000/240 =12.5A

(b)      V = IR

240=12.5R

R = 240/12.5 = 19.2 Ω

(c)      3 kW = 3000 W = 3000 joules per second

Energy used in 1 min = 3000 x 60 = 180 000 J

2        8A       31.25Ω

3        Resistance depends upon temperature

4        1 641 600 J

5        4800J

6        6

7        2.083 kW

8        1920W 1687.5 W

9        .64m

10      50kW  0.072W

Electric Field

Gravitation Field Lines

Gravitation Potential

1. -6.25 x 10^7 Jkg^-1
2.  -6.25 x 10^7 Jkg^-1 : -6.16 x 10^8 J : 1340 ms^-1
3. 1.91 x 10^11 Jkg^-1 ; 6.18 x10^5 ms^-1
4. 9.80 Jm^-1: 1.62 Jm^-1
5. 1.12 x 10^4 ms^-1 ; 2.37 x10^3 ms-1

G and Centrepetal force

Sun Earth

Angular Speed Satellite

Gravitational Field Strength

Electric Current

1. 36C
2. 500s
3. 12A
4. 66 weeks
5. 190mA  27 000C

SHM Revision